1. Field of the Invention
The present invention relates to a power amplifier for radio transmission which is used for a cellular phone and digital broadcasting, and more specifically to a predistorter which reduces the distortion of the power amplifier.
2. Description of the Related Art
A predistorter is available as a linear compensator for an amplification circuit having nonlinear characteristics. In a predistorter, the suppression of a distortion of an amplifier is realized, for example, by multiplying a coefficient according to the envelop of an original signal to be transmitted by the original signal as a predistortion coefficient for canceling the distortion of the amplifier. This predistortion coefficient, namely the distortion compensation coefficient is determined by the comparison of the original signal to be transmitted and the output signal of the amplifier.
FIG. 18 is a block diagram showing a first conventional configuration of a transmitter using such a predistorter. In FIG. 18, an input signal, namely an original signal to be transmitted is given to a multiplication circuit 101 provided in the preceding step of an amplifier 100. The predistortion coefficient stored in a predistortion coefficient table 102, namely the distortion compensation coefficient is multiplied and is inputted to the amplifier 100, and the output signal of the amplifier 100 is outputted via a directional coupler 103.
In regard to the reading of the data of the predistortion coefficient table 102, the envelope amplitude of an original signal is calculated, for example, by an envelope amplitude calculation unit 104, and the value of the amplitude is used as an address, and reference is made to the predistortion coefficient table 102.
Part of the output signal of the amplifier is given to a comparison computing unit 105 from the directional coupler 103; part of the signal and the input, namely the original signal are compared; a new distortion compensation coefficient is made by a coefficient making unit 106; and the predistortion coefficient table 102 is updated. In the coefficient making unit 106, for example, least mean square (LMS) is used as the algorithm for updating the coefficient.
FIG. 19 is a block diagram showing a second conventional configuration of a transmitter using a predistorter. In FIG. 19, the predistortion coefficient table 102 has a two-dimensional structure such that not only the value of the envelope amplitude but also the output of an envelope time differential calculation unit 107 which obtains the time differential of the envelope value or the time derivative of the envelope store the distortion compensation coefficient as an address, and the coefficient making unit 106 updates the distortion compensation coefficient using the calculation result of the envelope time differential calculation unit 107.
In regard to such a predistorter or a linear compensator, the following documents are available.    Document 1: Kokai (Jpn. unexamined patent publication) No. 54-140856 “Linear Compensator”    Document 2: Kokai (Jpn. unexamined patent publication) No. 9-69733 “Amplifier Having Distortion Compensation”    Document 3: Kokai (Jpn. unexamined patent publication) No. 2000-78037“Predistorter of Amplifier and Amplifying Device”.
Disclosed in Document 1 is a linear compensator described below. A nonlinear error between the input and output of an amplifier circuit is detected at all times; said error is written in a read/write memory as a digital signal; data in which a signal input level is written as an address is read as the amount of compensation; and a linear compensator traces the nonlinear characteristics of the amplifier at all times and can compensate them by controlling a programmable attenuator.
Disclosed in Document 2 is an amplifier having a distortion compensation function which can reduce the capacity of the distortion compensation table using least mean square (LSM) or exponentially weighted sequential least mean square (RLS; recursive least squares) as an applicable algorithm.
Disclosed in document 3 is a predistorter which transforms an input signal and makes a final predistortion signal based on the compensation coefficient corresponding to the value of the input signal of derivative or integral, or both of them in order to reduce the adjacent channel leakage power of the output power of an amplifier.
In a predistorter which compares an original signal and part of the output signal of an amplifier to determine a predistortion coefficient, there was a problem in that a correct predistortion coefficient cannot be determined if a frequency amplitude deviation exists (when frequency characteristics of amplitude are not flat), or if a delay error exists between the original signal and the output signal of the amplifier.
There was another problem in that it takes time to converge a predistortion coefficient, and an error becomes large in the region where the envelope amplitude of the original signal is small, the fluctuations of the envelope are large, and the absolute value of the differential at a certain time interval is large.
FIG. 20 to FIG. 22 show the problem when a frequency amplitude deviation exists and the envelope amplitude of the original signal is small. FIG. 20 shows two unmodulated signals (continuous wave signals: CW signals) used to explain this problem. The two CW signals shown in FIG. 20 can be indicated by following expression.cos(Δωt)cos(ωt)When such an original signal as shown in FIG. 20 is written on an IQ plane, it is illustrated as a thick line on an I plane as shown in FIG. 21.
However, when a frequency amplitude deviation exists, the input signal to the amplifier slips out of place and is expressed by an ellipse. Generally, the value of the envelope amplitude is expressed by the distance from the origin, so the original signal having the amplitude which should be shown in point A is shown in point B, and the value of the envelope amplitude becomes large.
FIG. 22 shows the distortion compensation coefficient which is made by the coefficient making unit 106 shown in FIG. 18 both when a frequency amplitude deviation exists and when no frequency amplitude deviation exists. When no frequency amplitude deviation exists, namely in the characteristics of the dotted line, an amplifier 100 has linear characteristics in the range in which the envelope amplitude as a reference value is small, and no distortion compensation should be required, thus making the value of distortion compensation coefficient 1. When the reference value becomes larger, the output of the amplifier 100 becomes saturated, so the value of distortion compensation coefficient to compensate the saturation becomes larger than 1.
On the other hand, when a frequency amplitude deviation exists, point A as an original signal is indicated as the signal of point B as described in FIG. 21, so the output of the amplifier 100 becomes larger than the value of linear characteristics if the value of distortion compensation coefficient remains to be 1. Consequently, a value smaller than 1 is obtained as a distortion compensation coefficient. The closer point A is to the original point, or the closer the reference value is to 0, the more rapidly the value of distortion compensation coefficient decreases at the positions in which the reference value is closer to 0. In such a case, as the result of multiplying the distortion compensation coefficient, a signal which becomes smaller like a spike in shape and is close to 0 is generated, thereby giving rise to a wide-band distortion.
As described above, when a frequency amplitude deviation exists, the predistorter outputs a distortion compensation coefficient which is close to 0 at the position in which the value of the envelope amplitude is small or the reference value is small. This is a natural action as a predistorter. When there exists no frequency amplitude deviation, a problem takes place in that this distortion compensation coefficient is multiplied to the original signal, so the input signal to the amplifier 100 becomes smaller, and the output error of the amplifier 100 becomes larger.
FIG. 23 to FIG. 25 show the problems in a range in which time delay exists in the output signal from an amplifier, and the absolute value of the time differential of the envelope value is large. In FIG. 23, the original signal is indicated by a thick line, and the waveform of time delay as the output signal of the amplifier is indicated by a thin line. When the size of the envelope becomes small in terms of time at a beginning position of FIG. 23, in other words, at the position in which the time differential value obtained by subtracting the envelope value at a prior time from the envelope value at a later time is minus, the signal which has passed through the amplifier, namely the value of the waveform of time delay looks larger than the value of the original signal, so the value of distortion compensation coefficient to correct this phenomenon becomes smaller than a correct value.
On the other hand, when the envelope value becomes large at the following position of FIG. 23, namely at the position in which the value of time differential is plus, the output signal of the amplifier looks smaller than the original signal, so the distortion compensation coefficient becomes larger than a correct value. Such an error becomes larger at the position in which the inclination of the signal is large or at the position in which the absolute value of time differential is large.
FIG. 24 shows the emergence frequency of the value of time differential and the value of envelope amplitude, using a CDMA (code division multiple access) signal as an example. From this figure, it is known that the emergence frequency is small at the position in which the absolute value of time differential is large.
FIG. 25 shows the spike-shaped error of the distortion compensation coefficient which emerges at the position in which the emergence frequency is small. Because the emergence frequency is small at the position in which the absolute value of time differential is large, as described in FIG. 24, it is considered that it takes time to converge the distortion compensation coefficient at such a position and that errors accumulate and spread. In such a case, a spike-shaped singular point of the distortion compensation coefficient emerges at only one position as shown in FIG. 25, and a problem takes place in that no correct distortion compensation effect can be obtained.
In view of the above-mentioned problems, the purpose of the present invention is to achieve the distortion compensation of a power amplifier using the correct value of a distortion compensation coefficient when a frequency amplitude deviation exists or the emergence frequency is small.
In other words, the purpose of the present invention is to compensate the distortion of an amplifier using the value of a correct distortion compensation coefficient when the value of envelope amplitude of an original signal is small, or at the position in which the time differential of the envelope value or the absolute value of time derivative is large.